The question we may ask is then, is there evidence in the data for systematic
differences along (or across) the luminosity sequence, which could hint to the
presesence, or even dominance, of a particular channel, at least in specific
luminosity/light-curve width ranges. This search does not appear to yield clear
results if it is performed based on early-time data. The temperature in the
line-forming region is a smooth function of luminosity
[1]}, [2]}. The luminosity distribution of SNe Ia, however,
seems to show two peaks: a main one, where all normal and moderately peculiar
SNe lie [3]}, [4]},
and a secondary, low-luminosity peak, which includes all rapidly declining SNe,
most notably the “cool” 1991bg class [5]}, [6]}, [7]}.
These two peaks appear to be quite distinct, with few transitional events
straddling in between [8]}. The low-luminosity group is not only
sub-dominant in number. It also shows a strong preference for passive galaxies,
suggesting that low-luminosity SNe Ia originate from older progenitor systems.
Simple evolutionary considerations suggest that scenarios involving DD systems
may be on average older than SD ones. Although exceptions are possible that can
alter this picture somewhat, the evidence for this remains circumstantial and
limited.
